1. Field of the Invention
The present invention relates to a via-hole processing method of applying a pulse laser beam to the rear surface side of a substrate of a wafer to form a via-hole reaching a bonding pad, in the wafer in which a plurality of devices are formed on the front surface of the substrate and bonding pads are formed on each of the devices.
2. Description of the Related Art
In a semiconductor device fabrication process, the front surface of an almost-disklike semiconductor wafer is sectioned into a plurality of regions by lattice-like arranged division-planning lines referred to as streets. Devices such as an IC, an LSI and the like are formed on the region thus sectioned. The semiconductor wafer is cut along the streets into the regions formed with the devices, thus fabricating individual semiconductor chips.
To promote downsizing and high-performance of a device, a module structure is put into practical use in which a plurality of semiconductor chips are stacked and the bonding pads of the stacked semiconductor chips are connected. This module structure is as below. A plurality of devices are formed on the front surface of a substrate constituting part of the semiconductor wafer and bonding pads are formed on the devices. A thin hole (via-hole) is bored at a position where each of the bonding pads is formed so as to reach the bonding pad from the rear surface side of the substrate. A conductive material such as aluminum, copper or the like is buried in the via-hole so as to be connected to the bonding pad. (See e.g. Japanese Patent Laid-open No. 2003-163323).
The via-hole formed in the above-mentioned semiconductor wafer is generally bored by a drill. However, since the via-hole formed in the semiconductor wafer has a diameter of as small as 100 to 300 μm, the boring by the drill is not necessarily satisfactory in terms of productivity. In addition, since the above-mentioned bonding pad has a thickness of about 1 to 5 μm, the drill must be controlled extremely accurately in order to form a via-hole in only a substrate made of silicon forming the wafer without breakage of a bonding pad.
To eliminate the above-mentioned problem, the present applicant has proposed a wafer-boring method, as Japanese Patent Application No. 2005-249643, of applying a pulse laser beam to the rear surface side of a substrate of a wafer to efficiently form a via-hole reaching a bonding pad, in the wafer in which a plurality of devices are formed on a front surface of the substrate and a bonding pad is formed on each of the devices.
The method of forming a via-hole by irradiation of a laser beam from the rear surface of the semiconductor wafer needs to bombard the same place with a pulse laser beam at a plurality of times. In addition, it is needed to stop bombardment just before an electrode called a bonding pad formed on the front surface of the semiconductor gets a hole. However, the energy distribution of a laser beam is a Gaussian distribution in which energy is strongest at the center and it is progressively reduced as it goes to the outer circumferential portion. The wafer is most processed by the central portion of the bombarding laser beam so that a laser processing hole having uniform depth cannot be formed. Thus, there arises a problem in that the bonding pad is melt to get a hole at a position corresponding to the central portion of the laser beam.